Mullions are vertical components that are frequently used in double door openings to separate the larger opening into two separate active openings. In certain circumstances, it is desired that the mullion be removable to enable the doorway to be used as a single larger opening, for example to accommodate the movement of large equipment through the doorway. Certain existing removable mullions are secured to the door using fasteners and/or a lock cylinder assembly, each of which may have limitations and/or drawbacks. For example, fasteners tend to become loose over time, do not prevent unauthorized persons from removing the mullion, and typically are more time-consuming to install and remove than keyed mullions. Certain mullions using lock cylinder assemblies can also be time-consuming to install and remove, and are typically more susceptible to vandal attacks that may damage the lock cylinder. For these reasons among others, there remains a need remains for further improvements in this technological field.